1. Field of the Invention
The present invention relates to an injector and more particularly to a structure for controlling a nozzle needle of the injector, which is driven to enable and disable inject fuel.
2. Description of Related Art
In an injector used in a common rail type fuel injection system of a diesel engine, a nozzle needle, which is driven to enable and disable fuel injection, is controlled by an actuator, such as a solenoid, to freely set fuel injection timing and an amount of fuel injection and thereby to achieve advanced fuel injection. One previously proposed injector includes a nozzle needle back pressure chamber, which exerts a back pressure of the nozzle needle upon supply of pressurized fuel (see, for example, Japanese Unexamined Patent Publication No. H08-49620). When the pressure of the nozzle needle back pressure chamber is increased or decreased, the nozzle needle is moved between a seated position and a lifted position relative to a valve seat. A release passage and a control valve chamber are formed in the injector. The release passage releases the pressure of the nozzle needle back pressure chamber to a low pressure source, and the control valve chamber forms an intermediate pat of the release passage. When a control valve, which is arranged in the control valve chamber, is driven to enable and disable communication between the nozzle needle back pressure chamber and the low pressure source, the pressure of the nozzle needle back pressure chamber is increased and decreased. The control valve is seatable against a seat formed in an outer peripheral part of a port of the control valve chamber, which is communicated with the nozzle needle back pressure chamber. The pressure of fuel of the port is applied to the control valve in a valve opening direction, and a spring force is applied to the control valve in a valve closing direction. When the solenoid attracts an armature, which is formed integrally with the control valve, the control valve is lifted against the spring force.
Here, the spring force is set to maintain the closed state of the control valve at the time of deenergizing of the solenoid. The required attractive force of the solenoid is determined based on the spring force.
When downsizing of the actuator (e.g., downsizing of the solenoid) needs to be achieved, the attractive force of the solenoid is also reduced due to a decrease in a magnetic surface area of the solenoid. Thus, the spring force should be also reduced, and the fuel pressure, which is applied to the control valve in the lifting direction, should be also reduced.
The fuel pressure, which is applied to the control valve in the lifting direction, can be reduced by sufficiently reducing a diameter of the seat of the control valve to reduce a pressure receiving surface area. However, due to the choking effect or throttling effect induced by reducing of the diameter of the seat, a pressure decreasing speed of the nozzle needle back pressure chamber may be excessively slowed to affect the responsibility of the nozzle needle. Furthermore, when an orifice is provided in the release passage to adjust the pressure decreasing speed of the nozzle needle back pressure chamber, an adjustable range is relatively narrow due to the above throttling effect. When the passage cross sectional area in the opened state of the control valve needs to be increased, the lift amount of the control valve can be increased. However, the attractive force of the solenoid valve is inversely proportional to a distance between the armature and the magnetic pole. Thus, in the case of increasing the lift amount of the control valve, a relatively large attractive force is required, and therefore the downsizing of the solenoid cannot be achieved.